1. Field of the Invention
The present invention relates generally to the field of semiconductor fabrication and more particularly to a mask used in the photolithography process during semiconductor fabrication.
2. Description of the Related Art
In the semiconductor industry, photolithography is used to transfer patterns corresponding to a circuit layout from a mask to a semiconductor wafer to form a semiconductor device. The layout, and hence the patterns on the masks, are made to conform to dimensional "design rules," which are determined by photolithographic and semiconductor processing parameters and circuit design criteria. Adhering to these design rules ensures that patterns on the mask transfer properly to the semiconductor wafer and ensures circuit functionality. One important design rule that determines the overall size and density of the device is the critical dimension, which is defined as the smallest width of a line or the smallest space between two lines.
Once the layout of the circuit has been created, an exposure tool is used to irradiate a layer of photoresist on the semiconductor wafer. An important limiting characteristic of the tool is its resolution. The resolution of an exposure tool is defined as the minimum feature that the exposure tool can repeatedly expose onto the wafer. As the critical dimensions of a mask layout approach the resolution limit of the lithography system, proximity effects begin to influence the manner in which features on a mask transfer to the resist layer such that the masked and actual layout patterns begin to differ. Proximity effects are known to result from optical diffraction in the projection system. This diffraction causes adjacent features to interact with each other in such a way as to produce pattern-dependent variations. One specific variation occurs when features are designed to have the same dimension but are placed in different proximity to other features in a layout. The difference in proximity causes features such as contact holes which are in close proximity to other features to print differently from features which are relatively isolated from other features. This variance between isolated features and closely-packed features is detrimental to process margin/latitude. As used herein, an isolated feature refers to a feature that is not appreciably affected during the photolighography process by proximity effects from other features, while a closely-packed feature refers to a feature that is appreciably affected during the photolithograph process by proximity effects from other features.
The proximity effects described above are not always detrimental. Diffracted exposure radiation from neighboring closely-packed features can actually interact to reduce sidelobe effects, thereby increasing depth of focus and hence process margin/latitude. Accordingly, and perhaps counter-intuitively, it is possible to achieve a smaller feature size and/or improved printing characteristics such as edge sharpness for closely-packed features than is possible for isolated features.
What is needed is a method and/or mask for, that can uniformly print both isolated and densely-packed features and that can improve printing of all features, whether closely-packed or isolated.